The mediaeval period lasted c. 1,000 years and Europe is a pretty large place so any generalisations will be open to contradiction.
The racial diversity per se is not the problem. The problem, it seems to me, is the way it is handled. To take three TV examples:
1. Game of Thrones. Different races occupied different regions - a sensible approach which didn't seem to bother many people. Books sold millions, TV much acclaimed (apart from it falling apart towards the end)
2. Rings of Power. Had the ridiculous situation whereby the population of a remote village had a demographic rivalling the General Council of the UN. And let us not forget the racially diverse hobbits. TV panned.
3. Wheels of time. The books have the same approach as GoT but the TV show has the same approach as RoP. Books sold millions, TV show panned.
The also Witcher suffers from this.
